1. Field of the Invention
This invention relates to an optical wavelength conversion device for converting an optical fundamental wave entering the device to an optical second harmonic having a wavelength one half the wavelength of the fundamental wave. This invention particularly relates to an optical wavelength conversion device using an organic nonlinear optical material.
2. Description of the Prior Art
Various attempts have heretofore been made to convert the wavelength of a laser beam to a shorter wavelength by utilization of optical second harmonic generation using a nonlinear optical material. As an optical wwavelength conversion device for carrying out wavelength conversion in this manner, there has heretofore been known a bulk crystal type device as described in, for example, "Hikari Electronics No Kiso" (Fundamentals of Optoelectronics) by A. Yariv, translated by Kunio Tada et al., Maruzen K.K., pp. 200-204. However, the bulk crystal type optical wavelength conversion device has the drawback that double refraction by a crystal is utilized for satisfying the phase matching conditions, and therefore a material which exhibits large nonlinearity, but exhibits no or little double refraction, cannot be utilized.
As one of optical wavelength conversion devices that eliminate the aforesaid drawback, a fiber type device has heretofore been proposed. The fiber type optical wavelength conversion device is constituted by an optical fiber comprising a cladding and a core which is formed of a nonlinear optical material and which is filled inward of the cladding. An example of the fiber type optical wavelength conversion device is described in Bulletin of Fine Optics Research Group, Meetings of The Japan Society of Applied Physics, Vol. 3, No. 2, pp. 28-32. The fiber type optical wavelength conversion device has the advantage that phase matching between the fundamental wave and the second harmonic can be achieved easily, and therefore extensive research has been conducted in recent years on the fiber type optical wavelength conversion device. Also, as disclosed in, for example, Japanese patent application Nos. 61(1986)-159292 and 61(1986)-159293, there has heretofore been proposed an optical waveguide type optical wavelength conversion device comprising two substrates as cladding layers, and an optical waveguide formed of a nonlinear optical material and disposed between the two substrates. The optical waveguide type optical wavelength conversion device also has the aforesaid advantage.
In recent years, it has been proposed to employ a single-crystal organic nonlinear optical material as the nonlinear optical material in the fiber type optical wavelength conversion device and the optical waveguide type optical wavelength conversion device. The organic nonlinear optical material has a nonlinear optical constant markedly larger than that of an inorganic material, and therefore a high wavelength conversion efficiency can be obtained by use of the organic nonlinear optical material. As the organic nonlinear optical material, it is possible to use, for example, 2-methyl-4-nitroaniline (MNA), methanitroaniline (mNA), 3-methyl-4-nitropyridine-1-oxide (POM), or urea as disclosed in Japanese unexamined patent publication No. 60(1985)-250334; "Nonlinear Optical Properties of Organic and Polymeric Materials", ACS SYMPOSIUM SERIES 223, David J. Williams, American Chemical Society, 1983; and "Yuki Hisenkei Kogaku Zairyo" (Organic Nonlinear Optical Materials), Masao Kato, et al., CMC K.K., 1985. As the organic nonlinear optical material, it is also possible to use 3,5-dimethyl-1-4-nitrophenyl)pyrazole, 3,5-dimethyl-1-(4-nitrophenyl)-1,2,4-triazole, 2-ethyl-1-(4-nitrophenyl)imidazole, 1-(4-nitrophenyl)pyrrole, 2-dimethylamino-1-5-nitroacetanilide, 5-nitro-2-pyrrolidinoacetanilide, or 3-methyl-4-nitropyridine-N-oxide as proposed in Japanese patent application No. 61(1986)-53884. For example, MNA has a wavelength conversion efficiency approximtely 2,000 times the wavelength conversion efficiency of LiNbO.sub.3 as an inorganic nonlinear optical material. Therefore, in the case where the optical wavelength conversion device is formed by use of the organic nonlinear optical material, a short wavelength laser beam of blue region can be obtained by generating a second harmonic from an infrared laser beam emitted as the fundamental wave from a popular small and low-cost semiconductor laser.
However, the fiber type optical wavelength conversion device obtained by forming the optical fiber core by use of the aforesaid organic nonlinear optical material or the optical waveguide type optical wavelength conversion device obtained by forming the optical waveguide by use of the aforesaid organic nonlinear optical material has the drawback that the wavelength conversion efficiency and the entry coupling efficiency of the fundamental wave deteriorate markedly with the passage of time. Specifically, the organic nonlinear optical material constituting the optical wavelength conversion device contacts the ambient atmosphere such as ambient air at its edge faces. Therefore, the organic nonlinear optical material sublimes from the edge faces to cause shortening of the single crystal part, or becomes modified from the single crystal condition. As a result, the aforesaid problems arise.